This invention relates to a high voltage generating power supply circuit for an X-ray tube, and more particularly to a power supply circuit of a voltage-resonance type.
Power supply circuits of a high frequency inverter type, which allow the use of a small size transformer, have widely been used for a high voltage power supply circuit for X-ray tubes. In this type of power supply circuit, a main switch and a subswitch are connected in series to each other in order to couple a primary winding of a transformer with a DC power source. A capacitor is connected in parallel to the main switch. When the main switch is open, the capacitor, together with the primary winding of the transformer, forms a series resonance circuits across the DC power source. Damper diodes are connected across the main switch and the subswitch, respectively. The secondary winding of the transformer is coupled with a bridge rectifier circuit. The bridge rectifier circuit is connected through cables to an X-ray tube. By operating the switches by a drive circuit, the primary current flows into the primary circuit of the transformer, so that a high voltage is produced in the secondary winding. The high voltage, 50 KV to 150 KV, is rectified by the bridge rectifier circuit, and supplied to the X-ray tube. The high voltage applied to the X-ray tube is adjusted by changing ON times of the main switch and the subswitch, while keeping the switching frequency constant. This control system is called a pulse modulation system.
The conventional power supply circuit of this type involves the following problems. Since the power supply circuit and the X-ray tube are connected by cables, resonance conditions of the resonance circuit greatly change with a rise in the tube voltage, that is, at the beginning of the operation of the drive circuit. The reason for this is that since cable capacitance is connected in parallel with the rectifier circuit, the secondary circuit of the power supply circuit is short-circuited due to the cable capacitance at the beginning of the operation. The transient phenomenon greatly disturbs the primary current of the transformer. As a result, energy is not smoothly transferred from the primary circuit to the secondary circuit of the transformer, and the rise of the X-ray tube voltage is slow.
Generally, X-rays emitted from the X-ray tube before the tube voltage reaches a desired voltage, does not contribute to the diagnosis. The slow rise of the tube voltage is accompanied by increases in the unnecessary radiation of X-rays and of the amount of X-rays radiated to a patient. Particularly, low energy x-rays radiated during the rise of the tube voltage (when the tube voltage is low) are liable to be absorbed by the human body. In this respect, the slow rise of the tube voltage cannot be ignored.
In the case of a power supply circuit for X-ray tubes with a tetrode circuit using a tetrode (high voltage switching four-element tube), the rise of the tube voltage is quick and free from the above-mentioned problem. The tetrode circuit, however, is large in size and weight, and costly.